Electrographic printing or copying devices (collectively called printing devices in the following) are known, for example, from EP-B1-0 683 954. According to FIG. 1, such a printing device comprises a driven intermediate carrier ZT, for example, a photoconductor drum, on which are generated charge images of the images to be printed that are inked with toner, subsequently transfer printed onto a recording medium AT (for example paper) and fixed to this in a fixing station FX. For this, the printing device comprises, for example, the following components grouped around the intermediate carrier ZT:                a charge corotron L to charge the intermediate carrier ZT,        an illumination device (character generator) DK,        a developer station E,        a transfer printing station UDS,        a cleaning station R,        an erasure corotron LSC,        a discharge lamp EDL.        
For example, the intermediate carrier ZT is charged to 500 V with the charge corotron L and then discharged to, for example, approximately 70 V with the illumination device DK to generate the charge images of the images to be printed. The charge images are subsequently inked with toner in a typical manner in the developer station E. However, the transfer of toner onto the intermediate carrier ZT only occurs when a sufficient voltage exists between the developer station E (for example a developer roller) and the discharged regions of the intermediate carrier ZT. If, for example, the developer roller exhibits a potential of 220 V and the charge images on the intermediate carrier ZT exhibit a potential of approximately 70 V, a field then results that pulls the toner from the developer station E to the intermediate carrier ZT. The toner images are subsequently transferred to the recording medium AT in the transfer printing station UDS. Finally the intermediate carrier ZT is cleaned of residual toner in the cleaning station R. The intermediate carrier ZT is prepared for a new printing event with the aid of the erasure corotron LSC and the discharge lamp EDL.
A developer made from carrier and toner (two-component developer) can be used in a known manner to develop the charge images on the intermediate carrier, whereby the fraction of toner is adjustable. In order to be able to check the fraction of toner, it is known (for example from WO 99/36834 A) to apply what is known as a toner marking that can be inked with toner onto the intermediate carrier. The inked toner marking can be scanned with a sensor that emits a signal dependent on the area coverage at the measurement location. This measurement signal serves to adjust the fraction of toner in the developer. The design of such a device and its operation can be learned from WO 99/36834 A, which is herewith included in the present disclosure. The toner supply into the developer station occurs in conveyer cycles that are implemented until the fraction of toner in the developer has reached the provided value.
A method for regulation of the toner supply in a developer station is known from U.S. Pat. No. 5,410,388 A. For this, a toner marking is generated and scanned on the intermediate carrier. When the scan of the toner marking results in that the signals generated in the scan of the front region and the scan of the rear region of the toner marking are different, and the signal associated with the front region is larger than the signal associated with the rear region, toner is conveyed into the developer station.
A method for control of the toner supply into a developer station is known from EP-A-0 546 953, in which the requirement for toner is determined and the scale of the supply of new toner per time unit is adjusted in the developer station dependent on this requirement. Differently designed dosing rollers are used for this. The requirement for toner is established either via counting of the image points of the images generated on the intermediate carrier or via measurement of the toner concentration in the developer station.
A method is known from U.S. Pat. No. 5,387,965 with which the toner supply to a developer station is controlled such that unwanted changes of the developer or of the intermediate carrier are taken into account. The toner concentration is thereby measured in the developer station by a sensor and compared with a predetermined reference value. Toner is conveyed into the developer station or not dependent on the comparison result.
A rule arrangement for a printing device is known from DE 38 07 121 A1, in which the developer station is controlled in order to achieve an optimal development of the charge images. The regulation occurs via a toner marking on the recording medium and its scanning. The toner supply to the developer station is regulated dependent on the scan result.
A further method for testing of the toner portion in the developer mixture with the aid of a toner marking can be learned from WO 00/41038 A.
Experience now shows that both the inking of the latent charge image on the intermediate carrier and the transfer printing of the print image from the intermediate carrier onto the recording medium are in particular impaired given printing operation with low toner throughput. Primarily responsible for this quality reduction is a deterioration of the developer that is created by mechanical friction forces given the mixing of carrier and toner. Two-component developer therefore requires a minimum degree of toner throughput in order to prevent the disadvantage cited above, since a too long mixing of the un-refreshed toner with the carrier in the developer station would lead to a damaging of the developer.
Given color printing, the problem of color drift additionally occurs in the case of the too-low print load, and therewith a too-low toner throughput.